Have I missed the boat?

Thinking over this old story of mine I get a little depressed. I finished it in 2004, showed it to someone well respected in the publishing world and was told something along the lines of 'King Arthur is old-fashioned and nobody wants to read about him now'. Then along comes Merlin and the rest, as they say, is history! Have I therefore missed the boat? (Grinds teeth and thumps her fist into her palm!) If I publish now will it seem like I'm jumping on the band wagon?

Who says the experts know what the next big thing is going to be?